TM 5-3805-261-20
PRINCIPLES OF OPERATION. (cont)
1-29.
HYDRAULIC SYSTEM. (cont)
c.
Hydraulic Control Valve (Figure 1-47). Each of the control levers at the sides of the steering console
operates a hydraulic control valve. The control valves are mounted side by side with the pump oil passages connected
and with the return oil passages connected so that only one line is needed for each. Each valve controls a hydraulic
cylinder or motor in the hydraulic system.
The operation of each control valve is the same. The rate of oil flow from each opening in a control valve for a hydraulic
cylinder is not the same. There are valves that have approximately the same rate of oil flow from each opening. The size
of the orifices from valve spool to openings is one of the controls for the rate of flow. The other control is the force of
spring against valve. The spring force is not the same in each control valve.
The resolver valve lets the oil with the higher pressure in either passage opening then through the valve into next passage.
The higher pressure of the oil on the ball in valve closes the valve passage with oil that has low pressure.
The implement valve in HOLD position has pump oil at a pressure of approximately 2150 psi in the valve when the engine
is running.
When the engine is started, the pump oil in passage goes into next passage and then through the holes in valve to the
chamber for the valve that has no spring. The pump oil in passage is stopped by valve spool and there is an increase in
the pressure of the oil from the pump. As the pressure of the oil gets higher, the oil pushes the valve against the force of
spring until valve is moved to a position where not much pump oil can go around valve into passage. There is a small
amount of oil leakage around valve spool in HOLD position because of the high pressure of the oil in passage.
The location of the implement valve puts the valve lower than the oil in the tank. Oil at tank pressure, through passages, is
in all passages in the valve that do not have pump oil. Air cannot get into the implement valve because of the oil in all of
the passages in the valve.
Figure 1-47.
1-44
